Pretty sure when they say it's airborne. They meant the virus are in droplets of our saliva carried in the air.
Hence the case where one infected choir singer infected 50 others when they all didn't wear masks.
The mask will filter the droplets such that it doesn't get into our respiratory system.
Air or gases however, i.e. oxygen, etc can still get through the mask including fart which is carbon dioxide, methane... gases. The virus doesn't float in the air by itself.
As they say it's chemistry my dear Watsons...